1. Field of the Invention
This invention relates to cathode structures for cathode ray tubes, and to a method for producing them, and more particularly relates to depression cathode structures produced from liquid cathode material and evidencing surface smoothness.
2. Prior Art
Cathode structures for cathode ray tubes desirably exhibit uniform electron emissions over an extended life cycle and under a variety of operating conditions. In addition, such cathode structures must be manufactured at the lowest possible cost. Because of such stringent requirements, particularly reliability and cost, there is great reluctance on the part of high volume manufacturers of cathode ray tubes to introduce new cathode structures or methods. Nevertheless, presently used cathode structures and methods exhibit limitations sufficiently troublesome to justify continuing investigations of alternate structures and methods.
These limitations include poor adherence of the emissive layer to its substrate, non-uniformity of emissions, and variations in the cathode-to-grid spacings (K-G.sub.1 spacings) of the electron gun, resulting in out-of-specification values for cut-off voltages. Both non-uniformity of emissions and variations in the K-G.sub.1 spacings can result from non-uniformity in the emissive layer. Particularly in the case of the sprayed coatings widely in use today, such non-uniformity can occur within a single cathode coating not only as thickness variation but also as surface roughness. Such variations in thickness and lack of surface smoothness can lead to variations in quality of the spot produced from the impingement of the electron beam on the phosphor screen. Such variations can also lead to high voltage arcing between the cathode and adjacent gun parts, which can contribute to shorter life of the cathode.
Adherence problems arise, particularly during operation near the high end of the normal temperature range, and can appear as lifting, flaking, or blistering of the emissive coating. Such adherence problems may be due in part to incomplete contact between relatively porous sprayed coatings and the underlying substrate. Such poor adherence can contribute to lower emission and shorter life of the cathode.
In addition to the above limitations, the spray coating process requires close control of a number of operating parameters, including air pressure of the spray gun, distance of the gun from the cathode, and the relative humidity of the atmosphere in the spray chamber, in order to achieve coatings of good quality. It is also necessary to spray each cathode up to 100 times in order to obtain the desired coating thickness. Such multiple spraying of cathodes of small area relative to the cross sectional area of the spray beam, results in the consumption of large quantities of the spray coating formulation.
"Depression" cathode structures are known in which cathode material is deposited within a cavity or depression in a cathode supporting structure. See, for example, U.S. Pat. Nos. 3,652,894; 3,148,056; 2,321,149; 2,929,133; and 2,716,716. However, such structures have not enjoyed wide acceptance in the manufacture of cathode ray tubes, possibly because of inherent difficulties in adapting such structures to mass production techniques. In addition, these prior art structures have not provided a cathode with controlled surface characteristics.
Accordingly, objectives of the present invention include: providing a cathode structure for cathode ray tubes which exhibits good adherence between the cathode layer and its supporting substrate; providing a cathode layer which exhibits both uniformity of thickness and surface smoothness; providing a cathode layer which exhibits a shaped surface; and providing a method for producing such structures simply and reliably.